docs(viewer): remove the HTML report skill

[Fluzko]

Part of #913. First of two stacked PRs; the second one fixes the remaining broken commands and references in the skills.

Problem

The dial9-html-report skill can't be followed by anyone who gets the skills through Symposium or dial9 agents skills. It asks the agent to build the viewer's web UI from a checkout of this repository and copy files out of it, and none of those files ship with the skills. It was also the only skill that ran the dial9 binary without saying where to get it.

What this PR does

• Removes the HTML report skill.
• Removes or rewords the places that still pointed at it (the UI README and two migration tickets).
• The dial9-zoom-window skill used the report's folder as its example of where a trace is served. It now uses the URLs that dial9 serve --local-dir actually serves, says where the dial9 binary comes from, and notes that the trace URL has to be percent-encoded inside a viewer link.

dial9 report serve is unchanged: it is a public command and still serves any report folder.

How it was checked

• cargo nextest run -p dial9-viewer skills_unpack_tests passes.
• The zoom-window URLs were checked against a running dial9 serve --local-dir.

[rcoh]

we should probably replace the html report with guidance about how to construct deep links

[Fluzko]

we should probably replace the html report with guidance about how to construct deep links

That's on #980. Since HTML report's job was handing findings to a person, I've tweaked skills that do the findings to build the URL leaning on the file that contains those instructions URL_CONTRACT.md. check 7481056

Pretty much what the report was doing is already covered, but distributed into different places/skills